In the courtyard ecosystems of the American continent, birds, as active participants, often face the need to transport multiple items such as nesting materials and food reserves. From the common house sparrow in North American backyards to the brown stove bird in South American backyards, different species have evolved transport abilities that are highly adapted to their survival needs. Behind these seemingly simple behaviors, there are complex decision-making mechanisms and physiological adaptations.
Core types of transported items and demand scenarios
The transportation behavior of birds in American courtyards mainly revolves around two core needs, and the physical characteristics of different items directly determine the differences in transportation strategies:
（1） Nesting material handling
The breeding season is the peak period for bird material transportation, and common nesting materials in the courtyard include twigs, grass stems, feathers, cotton wool, etc., with weights ranging from a few milligrams to tens of grams. Taking the Karoo reed wren in eastern North America as an example, its nest construction requires the transportation of over 1000 twigs, each weighing about 0.2 grams, with a total transportation capacity of more than three times its own weight. The handling of such materials usually follows the principle of "high frequency, small batch", and male birds will attract the attention of female birds by demonstrating efficient handling abilities during courtship.
For the handling of sticky materials, Mexican myxos demonstrate more precise adaptation. They use their beaks to peck at the wet mud in the courtyard and mix it with saliva to form moldable "building materials". Each time, they transport about 80% of their beaked volume to avoid flight imbalance caused by excessive weight. The brown stove birds in South America have elevated the handling of mud materials to an "engineering technology". They will travel dozens of times to collect wet mud and build oven shaped nests on sunny walls. The weight of a single mud mass transported can reach 15% of their own weight.
（2） Food resource transportation
During the non breeding season, the transportation of items is mainly focused on food reserves, with berries, seeds, and insect larvae commonly found in the courtyard being the main objects of transportation. The North American Blue Jays exhibit amazing food handling and storage abilities. In the autumn, they peck at the acorns in their yard one by one, hold them with their beaks, and fly to the storage location. They can carry 2-3 acorns per trip (with a total weight of about 5 grams), and their individual storage capacity can reach over 5000 throughout the autumn. This type of transportation behavior is accompanied by precise spatial memory, and studies have shown that the hippocampus significantly enlarges in autumn.
For the transportation of live prey, special techniques have evolved. After catching caterpillars, the Eastern Blue Robin uses its beak to hold the prey's head instead of its body, avoiding energy consumption caused by struggle; The larger American mockingbird, on the other hand, uses a "segmented drag" strategy when transporting earthworms. It first pulls its prey out of the ground and then adjusts its center of gravity by quickly flapping its wings to ensure that the elongated prey does not obstruct its flight.
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